Method of separating aromatic compounds from hydrocarbon mixtures containing the same by extractive distillation with an n-substituted morpholine



March 25, 1969 LUTHER ET AL 3,434,936

METHOD OF SEPARATING AROMATIC COMPOUNDS FROM HYDROCARBON MIXTURES CONTAINING THE SAME] BY EXTRACTIVE DISTILLATION WITH AN N-suBsTITuTED MORPHOLINE Filed D80. 18, 1967 United States Patent K Int. Cl. C07c 7/08; B01d 3/40 US. Cl. 203-32 11 Claims ABSTRACT OF THE DISCLOSURE Aromatic compounds present in either small or large proportions in a mixture of hydrocarbons are separated from such mixture by extractive distillation in the presence of an N-substituted morpholine having up to 7 carbon atoms in the substituent group and serving as a selective solvent for the aromatic compounds.

Background of the invention The present invention relates to a method of separating aromatic compounds from hydrocarbon mixtures containing the same, whereby the proportion of aromatic compounds in the mixture may vary greatly from relatively small to relatively very large proportions, and wherein the non-aromatic constituents in the mixture generally will include paraflins and/or cycloparaifins and/0r olefins, and also may include organic sulfur compounds. The separation of the aromatic compounds is carried out in accordance with the present invention by extractive distillation.

The method of the present invention is intended to serve for:

(a) recovering aromatic compounds of high purity from a starting material consisting essentially of a mixture of "hydrocarbons which may include greatly varying proportions of aromatic compounds, and

(b) for the removal ofaromatic compounds from a starting mixture of hydrocarbons which, in view of their later use-for instance in the food industry must be substantially freed of aromatic compounds.

In other words, the method of the present invention may be utilized in such cases where the purity of the separated aromatic fractions is the essential consideration, and also in such cases where the purity of the non-aromatic (parafiin) fraction is of prime importance.

The recovery of aromatic compounds from aromatic compounds-containing hydrocarbon mixtures by means of liquid-liquid extraction has been proposed for quite some time. For this purpose a great number of solvents 'have been proposed. However, a decisive disadvantage of these prior art methods of liquid-liquid extraction of aromatic compounds is found in the fact that most of the proposed extracting agents or solvents can be used only with the addition of water which is necessary in order to reduce the solubility of the non-aromatic compounds in the solvent or extraction agent and to adjust the selectivity of the solvent. However, this addition of water to the solvent requires during each distillation of the solvent the expenditure of correspondingly larger amounts of heat or energy and also requires special arrangements for the water circulation.

It has also been proposed to recover aromatic compounds by extractive distillation utilizing propylene carbonate as the selective solvent. In this case it is not 3,434,936 Patented Mar. 25, 1969 ICC proposed to adjust the selectivity of the solvent by the addition of water and it would also hardly be possible to operate with the addition of water. However, in extractive distillation processes, the selectivity of the solvent and its solubilizing characteristics are decisive prerequisites for the possibility of working up hydrocarbon mixtures of greatly varying content of aromatic compounds.

The last-mentioned method is excellently suited for working up hydrocarbon mixtures which contain a relatively large proportion of aromatic compounds, for instance so-called pressure raifinates which have aromatic compound contents of about 96%. However, the working up of hydrocarbon mixtures with relatively low content of aromatic compounds, for instance so-called reformates which contain between 4 and 10% benzene is possible by the last-mentioned method only if a heavy aromatic fraction is added to the propylene carbonate in order to increase the solubilizing action. This measure, however, reduces the selectivity of the propylene carbonate as a solvent and furthermore requires the circulation of large quantities of solvent in the extractive distillation arrangement.

-It is an object of the present invention to provide an extractive distillation process which, due to the specific selective solvent utilized, will make it possible to work up hydrocarbon mixtures within the entire technically accruing range of concentration of aromatic compounds, either so as to obtain pure aromatic fraction, or to remove residual aromatic compounds from a pure non-aromatic or paraflinic fraction.

Summary of the invention The present invention proposes to separate aromatic compounds from a mixture of hydrocarbons which consists partly of aromatic compounds, by subjecting the mixture to extractive distillation in the presence of a selective solvent for the aromatic compounds which selective solvent is an N-snbstituted morpholine which has up to 7 carbon atoms in the substituent group, whereby an extract will be obtained which consists essentially of the aromatic compounds dissolved in the substituted morpholine compound. The thus-obtained extract may then be recovered and if the original hydrocarbon mixtures included unsaturated compounds which partly may have been incorporated in the extract, the extract may then be subjected to hydrogenation.

The recovered extract may then be freed of solvent, the solvent separately recovered by distillation and, if the original mixture contained unsaturated or heterogeneous compounds, it will frequently be desirable to subject the recovered extract prior to distilling-off the solvent, to washing with sulfuric acid or to treatment with a bleaching earth.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

The single figure of the drawing is a schematic elevational representation of an arrangement for carrying out the method of the present invention.

Description of the preferred embodiments According to the present invention, an aromatic cornponuds-containing mixture of hydrocarbons is separated into aromatic and non-aromatic compounds by extractive distillation utilizing as the selective solvent for the aromatic fraction one or more N-substituted morpholines which contain no more than 7 carbon atoms in the substituent.

It is not desirable to use as the solvent substituted morpholines which contain more than 7 carbon atoms in the substituent because such compounds with more than 7 carbon atoms in the substituent have a too high molecular weight and in such case the amount of solvent or extraction agent which has to be utilized would have to be unduly increased. However, it would seem possible, although uneconomical, to carry out the process of the present invention also with substituted morpholines having more than 7 carbon atoms in the substituent group.

The substituents may be of straight chain, branched or ring-shaped structure. Generally, when utilizing N-substituted morpholines wherein the substituent group is of straight chain structure, it will be preferred to utilize substituent groups with not more than 4 carbon atoms. The N-substituted morpholines may be used either individually or in the form of mixtures thereof.

The solvents which are utilized in accordance with the present invention, i.e., the N-substituted morpholines with not more than 7 carbon atoms in the substituent group, all have more or less the same boiling range; however, they differ with respect to their solubilizing action and their selectivity as a solvent. By utilizing suitable mixtures of such N-substituted morpholines, solubilizing conditions can be adjusted which are geared to the concentration of aromatic compounds in the starting mixture of hydrocarbons which is to be worked up. This can be done without reducing the selectivity of the solvent mixture as would be the case by adding a heavy or high molecular weight aromatic compound in accordance with the prior art described further above. It is an additional advantage of the method of the present invention that relatively small amounts of solvent have to be recirculated due to the fact that it is not necessary to add heavy aromatic compounds for carrying out the extractive distillation.

The adjustment of the desired proportion of substituted morpholines for forming a solvent mixture which is best suited for the specific purpose at hand is well within the skill of the art and generally it may be stated that if the starting mixture contains a high proportion of cyclic hydrocarbons such as aromatic compounds and cycloparaffins, it will be preferable to utilize as solvent or extraction agent compounds of strongly polar characteristics such as N-formylmorpholine. However, if the starting mixture contains only a relatively small proportion of cyclic hydrocarbons, extration agents of less polar character may be utilized, for instance N-oxyethylrnorpholine.

According to a further feature of the present invention, the extractive distillation is preferably carried out without reflux of hydrocarbons which have been distilled off. In other words, condensate is not added to the raffinate phase at the head of the extractive distillation column. The column thus works only with so-called interior reflux which adjusts itself by condensation of extraction agent or solvent within the column.

The last discussed feature of the present invention, namely the carrying out of the extractive distillation without refluxing distilled-off hydrocarbons which have been condensed after leaving the distillation column would seem to be surprising since it is the general consessus of the art that refluxing to the largest technically feasible amount is desirable. However, it has been found that these teachings of the general distillation technique cannot be directly applied to extractive distillation systems.

The reflux which is added at the head of the extractive distillation column mixes with the extraction agent or solvent and reduces the selectivity of the latter. Furthermore, it also requires to again vaporize the refluxed hydrocarbons and this increases the energy demands of the system. This again requires that the sump of the extractive distillation column must be heated more strongly so that also a correspondingly higher proportion of the aromatic compounds located in the sump will be vaporized. Since these vapors also have a correspondingly higher temperature, they will rise higher in the extractive distillation column before they will be condensed again. This also impairs efliciency or completeness of the separation of the aromatic compounds. It is a further disadvantage of allowing reflux of condensed hydrocarbons to be introduced into the extractive distillation column that the higher heating of the sump area of the column which is required for revaporization of the reflux will favor the formation of polymerizates in the sump. This is particularly marked if the starting mixture contains olefins and/ or diolefins. The thus formed polymerizates will cause foam formation within the extractive distillation column and this again will impair the separation of the aromatic compounds.

If the starting material, i.e., the starting mixture of hydrocarbons, contains unsaturated and/or heterogeneous, i.e. heterocyclic, compounds such as cyclopentadiene or thiophene, it is frequently desirable to subject the extract obtained in the extractive distillation, i.e. the solution of aromatic compounds, to washing with sulfuric acid or treatment with bleaching or Fullers earth.

The regeneration and recovery of the selective solvent or solvent mixture is carried out in per se known manner by distillation. If the solvent is contaminated with unsaturated compounds which may have been originally contained in the starting material, it is preferred to carry out regeneration of the solvent by first hydrogenating the same, for instance in the sump of the regenerating col umn, followed by distillation.

Hydrogenation of the unsaturated compounds-containing phase, for instance by means of a conventional nickel catalyst, is connected with the further advantage that thereby unsaturated hydrocarbons which may be contained in the solvent as impurities will be partially hydrogenated or subjected to hydrogenating cracking. For instance, dicyclopentadiene will be decomposed into 2 moles of cyclopentane. The saturated hydrocarbons which are formed by the hydrogenation in the sump phase can be easily separated from the solvent upon recycling of the latter into the extractive distillation column.

The following examples are given as illustrative only without limiting the invention to the specific details thereof.

The examples refer to the working up of hydrocarbon mixtures of varying compositions in an arrangement such as illustrated in the drawing.

Referring now to the drawing, the extractive distillation column 1 may be a bubble tray column with sixty trays. The starting product is introduced through conduit 2 into the middle of the column, for instance at the twenty-first, thirty-first or forty-first tray counted from the top of the column. The non-aromatic compounds are withdrawn from the head of the distillation column through conduit 3. Recycled solvent is introduced through conduit 4 at the uppermost tray of column 1. However, as pointed out further above, only recycled solvent and no condensed hydrocarbons are reintroduced into column 1. The aromatic compounds and the solvent carrying the same are withdrawn through column 5 at the sump or bottom of column 1 and conveyed to stripper 6 which also consists of a bubble tray column with forty trays. Introduction of the extract consisting of the aromatic compounds dissolved in the solvent is carried out at the fifteenth tray counted from the top of column 6. The pure aromatic compounds are withdrawn through conduit 7 at the head of column 6 or a few trays below the head.

If indicated for the reasons discussed further above, the aromatic compounds may be, prior to being finally withdrawn through conduit 15, subjected to a sulfuric acid or bleaching earth treatment in a suitable treating vessel 8 of per se conventional structure. Furthermore, if necessary, the higher boiling aromatic compounds may be conveyed through conduit 9 to stripper 10. Stripper column 10 serves for further purification of the higher boiling aromatic compounds which were withdrawn through conduit 9. These higher boiling aromatic compounds will accure and be concentrated in the bottom portion of stripper column 10 whereas any lower boiling constituents may -be withdrawn at the head of stripper column or, if desired, recycled into stripper column 6.

The withdrawal of the pure final product, i.e., the pure aromatic compounds, is carried out through conduit 11. The solvent which has thus been freed of aromatic compounds is withdrawn at the bottom of column 6 and recycled through conduit 4 to extractive distillation column 1. A partial stream of the distilled-off solvent may-if necessary be introduced through conduit 12 at one of the intermediate trays of regenerative column 13. At the bottom of column 13, nondistillable impurities may be withdrawn through conduit 17, whereas the solvent may be withdrawn at the head or two trays below the head of regenerative column 13 and passed through conduit 14 into conduit 4 and into the recirculation system.

If the starting material contains olefins, the regenerating column 13 may also serve as a pressure container for hydrogenation, in which case the trays may include catalyst beds so that the regenerative column 13 will also serve for hydrogenation of the impurities particularly non-distillable impurities. It is also possible to carry out distillative regeneration and the above-described hydrogenation in the bottom portion of the regenerative column 13 in simultaneous manner. Conduit 16 serves for introducing hydrogen gas required for the above-described hydrogenation into regenerative column 13.

The data for the following four examples are given TABLE II-Continued Reference numeral Percent by weight 11-03 hydrocarbons 0. 06 0.23 0.01 Isa-C9 hydrocarbons" 0. 04 Toluene 14.90 2 83 n'Ct hydrocarbons Ca hydrocarbons.-.

Isa-G hydrocarbons.

n-C7 hydrocarbons Isa-g Chydrocarbons up to boiling point poin l1 n-0 hydrocarbons Iso-C hydrocarbons Toluene below in table form, whereby Table I will serve to de- Xylenes- Solidifioatwn point scribe the essential operative data.

TABLE I Example Number Starting material Crude benzene Reiormate Pyrolysis- Benzine traction benzlne Extraction agent (solvent) N-formyl- N-oxyethyl- N -acety1- N -phenylmorpholine morpholine morpholine-lmorpholine N-iormylmorpholine Amount of starting material, kg 52 52 52 52 Starting temperature, C 58 58 58 58 Temperature at head of column 1, C 56 69 Amount of non-aromatic fraction at conduit 3, kg 3. 5 32 25. 2 49. 8 Solvent amount, kg 207 280 292 142 Temperature at bottom portion of column 1, C 124 159 180 206 Temperature at head of column 6, C 80 8O 30 Amount of aromatic fraction at conduit 7, kg 40, 7 5.2 13. 7 2. 2 Amount of heavy aromatic compounds at conduit 11, kg 7. 8 14. 9 13. 1 Temperature at sump or button portion of column 6, C 201 230 210 158 The most important analytical data for the four examples are summarized in Tables II-V. At the head of each of the tables the respective portion of the distillation arrangement from which the analyzed material was taken is indicated by the corresponding reference numeral of the drawing.

TABLE II.ANALYTICAL DATA FOR EXAMPLE NUMBER 1 Reference numeral Percent by weight C4-C hydrocarbons Isa-On hydrocarbons. n-CQ hydrocarbons. Cyclopentadiene Methylcyclopentane and iso-C1 hydrocarbons Oyolohexane--- Heptane-.. Thiophene 0 Isa c; hydrocarbons up to boiling point C. Benzene 78.30 Methylcyclohexane 0. 43 ISO-Cs hydrocarbons starting with boiling point 115 C 0. 04

TABLE IV.ANALYTI CAL DATA FOR EXAMPLE NUMBER Reference numeral Percent by weight 05 hydrocarbons 14v 0 27. 8

Ca hydrocarbons plus cyclopentane. 23. 7 47.1 Isa-C1 hydrocarbons plus methylcyclopentane 4. 2 n-Heptane Cyclohexane Iso-Ca hydrocarbons up to point 115 0. plus methylcyclohexboiling point 115 C n-Cs hydrocarbons l. ISO-C9 hydrocarbons plus dimethylcyclohexane Toluene 5.51 C. Norm-The non-aromatics contained varying amounts of olefins.

TABLE V.ANALYTICAL DATA FOR EXAMPLE NUMBER 4 Reference numeral Percent by weight As shown in the examples above, the method of the present invention permits in a simple and economical manner to separate aroma-tic compounds from the respective hydrocarbon mixtures.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. Method of separating aromatic compounds from a mixture of hydrocarbons consisting partly of aromatic compounds and also including non-aromatic hydrocarbons, comprising the steps of subjecting said mixture in an extractive distillation means to extractive distillation in the presence of a selective solvent for said aromatic compounds, said selective solvent being selected from the group consisting of N-substituted morpholines having up to 7 carbon atoms in the substituent, so as to obtain extract bottoms consisting predominantly of said aromatic compounds dissolved in said selective solvent and simultaneously to distill-off non-aromatic hydrocarbons; recovering said extract bottoms consisting predominantly of said aromatic compounds dissolved in said selective solvent; and recovering the distilled-off nonaromatic hydrocarbons outside the extractive distillation means in the absence of refluxing said distilled-off nonaromatic hydrocarbons.

2. A method as defined in claim 1, wherein said mixture includes non-aromatic compounds selected from the group consisting of parafiins, cycloparaffins, olefins and diolefins.

3. A method as defined in claim 2, wherein said mixture includes organic sulfur compounds.

4. A method as defined in claim 2, wherein said selective solvent is a mixture of at least two of said N- substituted morpholines.

5. A method as defined in claim 2, wherein the recovery of the distilled-01f non-aromatic hydrocarbons comprises condensing the distilled-oif non-aromatic hydrocarbons outside the extractive distillation means.

6. A method as defined in claim 2, wherein said mixture of hydrocarbons includes unsaturated or heterocyclic compounds, and said recovered extract bottoms are subjected to washing with sulfuric acid.

7. A method as defined in claim 2, wherein said mixture of hydrocarbons includes unsaturated or heterocyclic compounds, and said recovered extract bottoms are treated with Fullers earth.

8. A method as defined in claim 2, and including the step of recovering said solvent from said extract bottoms.

9. A method as defined in claim 8, wherein said recovering of said solvent is carried out by distillation.

10. A method as defined in claim 2, wherein said mixture additionally includes unsaturated compounds which at least partly accrue in said extract bottoms, and including the step of subjecting said unsaturated compoundscontaining extract bottoms to hydrogenation.

11. A method as defined in claim 10, wherein, after said hydrogenation, solvent is recovered from said extract bottoms by distillation.

IV, Distillation, New York 1965, Interscience PebL, pp. 457-462 relied upon.

WILBUR L. BASCOMB, JR., Primary Examiner.

US. Cl. X.R. 

